Offshore data acquisition for well and gas exploration is generally performed by means of vessels which pull seismic streamers below the surface of the water. These marine seismic streamers are generally 3000 to 15000 meters in length and are towed at a depth of about 20 to 60 feet. The streamers include hundreds of hydrophones or other seismic sensors which detect seismic waves that are reflected from a shallow sound source, through the sub-bottom layers, and back up to the hydrophones. These waves are then transmitted to the towing vessel through the streamer where they can be processed to obtain information concerning underwater formations.
Remote controlled cable levelers (or bird devices) are often attached to a streamer to keep the streamer level and to control its depth. These levelers include wings which can be adjusted to cause the streamer to rise or dive as it is pulled through the water. Generally 10 to 15 cable levelers are needed to maintain adequate control of the streamer.
Remote controlled compasses are also secured at various locations along the length of the streamer to provide a means for determining the position of the streamer. These compasses transmit heading data back to the seismic vessel where it can be utilized to calculate the position of the streamer.
The streamer, cable levelers, and compasses comprise expensive pieces of electronic equipment. The cost of a single streamer can be in the hundreds of thousands of dollars. Seismic streamers are frequently torn into pieces or are pulled entirely from the seismic vessel by obstructions in the water such as oil and gas rigs, crab pots, fishing lines, etc. Because of the high cost of the streamers, it is highly desirable to retrieve severed streamers so that they may be repaired and reused.
Seismic streamers are generally filled with a light oil, gel or solid ballast material to provide neutral buoyancy along the length of the streamer. Accordingly, when a streamer is severed from the towing vessel, it does not readily rise to the surface nor sink to the bottom. Rather, the streamer gradually changes depth depending upon the salinity of the water and any currents. Thus, the momentum of the cable from being towed and ocean currents can carry the streamer for significant distances with its depth changing only gradually or in some circumstances changing significantly. Thus, it is extremely difficult to locate the position of a severed streamer.
The current procedures which are utilized to retrieve marine seismic streamers are generally unsatisfactory. While it is possible to put transmitters in a streamer to provide a means for locating the streamer once it is severed, it is still difficult to retrieve the streamer from the water. It is necessary for a diver to physically locate the streamer and to attach some type of cable or floatation device to bring the streamer to the surface.